[Sethomas]

An MRI operator will not run a scan on you with metal piercings. Any prosthesis, pace maker, whatever has to be identified by model number so that the alloys can be scrutinized for magnetic susceptibility. A good rule of thumb, though, is that anything with the word "steel" in it shouldn't be near an MRI.

Also, what most people don't realize is that the magnetic field of an MRI is so uniform that the danger is not having metal ripped out of your body (in most cases). The danger is that metal in a B-field induces current (the Lorentz force, I think?) in moving metals, which is usually just manifested into extreme heat. Evidently there have been cases of tool workers who gradually had a build-up of iron filament in their eyes, and upon getting an MRI they were introduced to a painful lesson in E/M physics.

I think the physics behind MRIs are pretty cool. On paper, I think that PET scans are pretty awesome, but the idea of putting chemicals into a living organism's body so that decaying isotopes will emit anti-matter to collide with bodily valence electrons so that detectors can analyze the gamma-frequency photons that get shot out, well, just seems like a bad idea.